1. Field of the Invention
The present invention relates to a tag communication system, an interference preventing method and a tag communication controller suitable for prevention of a radio wave interference able to be caused by radio waves mutually emitted when plural tag communication devices are arranged.
2. Background Art
When plural reader-writers as tag communication devices are arranged and wireless communication is performed through a RFID tag attached to baggage and an antenna, there is a case in which radio waves mutually emitted interfere and cause an obstacle of the communication. As a method for preventing this interference, there are formerly a method for preventing the interference by synchronizing signal transmission timing of the reader-writer, a method for preventing the interference by adjusting a direction of the antenna and arranging an absorbing body, a method for normalizing a new interference preventing function such as LBT (Listen Before Talk) and DRM (Dens Read Mode), etc.
However, in these methods, there are problems such as a reduction of a system speed due to wiring between the reader-writers and time division communication, arranging cost and a reduction of a freedom degree of the arrangement, a reduction of the system speed due to the time division communication, an increase of a required frequency band.
Therefore, for example, there is a technique disclosed in JP-A-2006-42268 as an interference preventing method replaced with these methods. In the following description, reference numerals within parentheses are described in JP-A-2006-42268. The technique disclosed in this JP-A-2006-42268 reduces an interference from other electronic authenticating devices in an environment for arranging plural electronic authenticating devices. For example, this JP-A-2006-42268 discloses a show case for arranging two element antennas (12a, 12b) as shown in FIGS. 8 and 9 (views corresponding to FIGS. 7 and 8 of JP-A-2006-42268). This JP-A-2006-42268 discloses a method for preventing an interference between reader/writer when the RFID reader/writer constructed from a main body (10), an electricity supply network (11), plural element antennas (12a, 12b) and a personal computer (13) is arranged every show case as shown in FIGS. 8 and 9. Concretely, synthetic directivity of the antenna constructed by two element antennas (12a, 12b) is changed by changing the amplitude and phase of an electricity supply signal given to the element antenna (12a). The interference between the reader/writer is reduced by directing a null in a direction of the show case of a partner side, i.e., by directing a zero point or a low sensitivity portion between lobes of directivity of the antenna.
However, in JP-A-2006-42268, the element antennas (12a, 12b) are arranged within the show case. Accordingly, for example, when an article of commerce is taken in and out, and this element antenna (12a, 12b) comes in contact with a person's hand and is moved and a shift is caused in the directivity of the radio wave, a situation for directing no null to an interference wave is caused. As a result, an interference can be caused. Accordingly, it cannot be said that it is sufficient as the interference preventing method. Further, when the reader-writer used for a purpose except for management of the show case is newly arranged, there is a possibility that a new interference is generated and an obstacle is caused in the system.